This research establishes 5 mm three-dimensional (3-d) flow and heat transfer
microfin tube theoretical models with three different geometric structures.
Using these models, the thermal-hydraulic performances of supercritical
CO2/R32 in microfin tubes with different structures at various working
conditions were investigated. The influences of each of three factors
(pressure, mass flow, and microfin tube structures) on the thermal-hydraulic
performance of CO2/R32 were evaluated respectively. Furthermore, orthogonal
tests were undertaken to obtain the optimized combination of overall
thermal-hydraulic performance. Results indicate that: the more the
temperature of working media approximates to the critical temperature, the
bigger the local convective heat transfer coefficient. Compared to
non-critical temperatures, the convective heat transfer coefficient at
critical temperature shows an eight-fold increase. The closer the pressure
of the mixed working media is to the critical pressure, the greater the
maximum convective heat transfer coefficient (CHTC) and the lower the
temperature corresponding to the peak point, among which, the maximum CHTC
under 7.5 MPa is three times as large as that at 8.5 MPa; the CHTC increases
with increasing mass velocity, generally showing a linear relationship;
through calculating the most optimal combination of thermal-hydraulic
performance evaluation using orthogonal tests, the maximum CHTC is
determined to be 96 kW/(m2?K).